Digital communication systems typically organize transmissions into blocks of symbols, according to a preselected protocol. A transmitter is used to transmit the blocks in a predetermined frequency band or channel to a receiver. However, the channel may have transmissions according to more than one protocol. The receiver must then monitor the channel for transmissions according to the proper protocol to receive and process a block. FIG. 1 is a simplified functional block diagram of such a digital communication system 10. Although not shown for clarity, as is well known, receiver 14 also includes filters, an analog-to-digital converter, demodulator, etc. In exemplary system 10, a transmitter 12 is capable of transmitting blocks according to several different protocols, including protocols P1, P2, and P3. In this example, all three of the protocols may be transmitted in a single channel at different times.
FIG. 2 shows a sequence of blocks transmitted according to different protocols in a single channel. As a result, a receiver 14 monitoring the channel will receive blocks according to all three protocols. However, in some communication systems, the receiver can process transmissions according to only one protocol. For example, in a paging system, the pagers carried by the users generally can process pages according to only a single protocol (e.g., POCSAG). In this example, receiver 14 includes a protocol detector 16 to determine if a detected transmission conforms to the receiver's protocol. Some conventional protocol detectors use correlation techniques to identify the block's protocol.
More specifically, as shown in FIG. 2, each block has a synchronization segment or header synchronization (header sync) pattern. Protocol detector 16 then correlates the header sync pattern of its protocol continuously to the received signal, as indicated by correlator 18. For example, in a transmission according to protocol P.sub.1, the transmitted block includes a synchronization or header sync portion S.sub.1 followed by a data portion D.sub.1. Header sync portion S.sub.1 typically includes a preselected sequence or pattern of symbols that are unique to protocol P.sub.1. Similarly, in a transmission according to protocol P.sub.2, the transmitted block has a unique header sync portion S.sub.2 and a data portion D.sub.2, and so on. When the received signal contains a header sync pattern that aligns with the correlator according to the receiver's protocol, the correlation output signal level is relatively high. Thus, a high level of the correlation output signal is indicative of a matching header sync. However, this type of protocol detector is susceptible to signal-to-noise (SNR) problems caused by a frequency offset in the local oscillator (LO) of the receiver. If a longer sync pattern is used to overcome the degraded SNR caused by the frequency offset of the LO, the detection process becomes more susceptible to signal changes due to fading. Also, the correlation output is degraded by the filtering of the signal by the channel impulse response. Accordingly, there is a need for a protocol detector that is relatively insensitive to frequency offsets, low SNR, and channel impairments such as multipath fading.